There has so far been used a computer system provided with a normal or operating site and a standby site for maintaining the system function even on outbreak of disasters. Such computer system is termed a replication system. During normal operation time, the normal site is performing operation for providing system functions and, in case the normal system is unable to operate as regularly, the standby site comes into operation to take the place of the normal site.
For providing the functions as the computer system, the operating site and the standby site have respective storage systems for storing data. In the replication system, data in the storage system of the operating site is replicated and retained in the storage system of the standby site, such that the standby site may come into operation to take the place of the operating site (for example, see Patent Documents 1 and 2). This processing is termed the ‘replication’.
In the replication system, the normal site and the standby site may be ‘synchronized’, in which case the replication is termed ‘synchronous replication’. Or, the normal site and the standby site may be ‘non-synchronized’, in which case the replication is termed ‘asynchronous replication’.
If, in the synchronous replication, there occurs Write in a storage device in the normal site, the same data is written in the storage device of the stand-by site, with the Write for the normal site acting as a trigger. In the asynchronous replication, WRITE in the storage device of the stand-by site is carried out subsequently, with the WRITE for the normal site then not acting as a trigger.
Among the methods currently known for decreasing the band of the network, there is a method in which only the data updated latest in the storage is transmitted as update data for the same block (Non-Patent Document 1). The amount of data traffic over a network may be reduced more than in case of complete data copying by transferring and copying only the difference (‘gap’) between the data stored in the normal system and that stored in the standby system. However, in these systems, the amount of data transfer may be decreased only when update has occurred for the same area during predetermined time duration. The amount of data transfer cannot be decreased for data update for physically or logically differing areas.
In a system for implementing the replication with a middleware layer, such as a database, there is known a method which resides in transferring a REDO log to create a data file in the standby site (Non-Patent Document 3). If replication is used even with this system, data not in need of being sent to the stand-by site do not have to be sent, thereby diminishing the amount of data transferred on the network. However, there arises the necessity for a computer capacity for running the middleware, software or software license in the stand-by site. In addition, there is a possibility that the application run on the normal site may be lowered in performance as a result of the middleware or the replication function in the middleware being run on the host. There is also presented a problem that the application being run has to be developed using the middleware, and hence the system has to be modified for newly introducing the middleware in mirroring a pre-existing system.
Meanwhile, as a prior-art publication, the Patent Document 1 shows a method and a system for reducing the amount of data transfer necessary for copying DB data to assure data matched as DB table of the destination of copy. It is stated that, with use of the log, data to be copied to a remote device may be buffered with the amount of information less than with buffering on a cache. However, with the method and the system disclosed in Patent Document 1, if data of the same contents are written in plural distinct locations, it becomes necessary to transmit data per se.
In Patent Document 2, there is disclosed a system in which a frame from a central processing unit is temporally stored in a header converting buffer in a master side disc apparatus and in which the header part of the frame is converted into a header part for a remote side disc apparatus, and the resulting frame is then transmitted.
In Patent Document 3, there is disclosed the configuration in which copying means in a concentrator in a disc array device of a primary site copies update data on a cache in a manner of sequentially packing the data in a transmitting buffer from the leading end, a transmitting means transfers the packing data of the storage buffer to a disc array device of a secondary site, and in which a receiving means in the disc array device of the secondary site copies update data in the packing data in a multiplexed form in the packing sequence. In this case, the update data are transmitted in their entirety.
In Patent Document 4, there is disclosed a system reducing the amount of communication in multiplexing a master file and a duplicated file. In this system, a master file read in from a master area is cached in a cache area, a changed data part in the cache area is transferred to an associated part in a master area, and the changed data part is multiplexed in the master area and in the cache area, while the non-changed data part is multiplexed in the master area and the copying area. Finally, the non-changed data part is transferred to the cache area and the duplicated data is integrated in the cache area to provide a duplicated area. It is sufficient to transfer data only to the master area at the time of preparation of the duplicated file, and hence the volume of communication may be reduced for multiplication.
[Non-Patent Document 1]
EMC Corporation, EMC SRDF, SRDF/A [ONLINE] (Retrieved on Jul. 28, 2004, Internet <URL http://japan.emc.com/local/ja/jp/products/networking/srdf.jsp>
[Non-Patent Document 2]
NEC Corporation, SYSTEM GLOBE REMOTE DATA REPLICATION [ONLINE] [Retrieved on Jul. 28, 2004], Internet <URL http://www.sw.nec.co.jp/products/istorage/product/software/rdr/index.s html>
[Non-Patent Document 3]
Oracle, ORACLE DATA GUARD 10G White Paper [ONLINE] [Jul. 28, 2004] Internet <URL: http://otn.oracle.co.jp/cgi-bin/otn/auth_r.cgi?path=/download/products/database/oracle10g/pdf/twp_ha_dataguardoverview—10gr1—1103.pdf>
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2000-347917A
[Patent Document 2]
JP Patent Kokai Publication No. JP-P2003-085017A
[Patent Document 3]
JP Patent Kokai Publication No. JP-P2003-167684A
[Patent Document 4]
JP Patent Kokai Publication No. JP-P2003-323328A